Found it at
https://chauthanh.info/animeost/album/under-night-in-birth-exe-late-st-original-soundtrackYou'll probably need some software for downloading from multiple links as the site doesn't allow for that without paying.
Something like JDownloader2, the main download has adware, but I think there's a clean (no adware) installer on their forums. I haven't used JDownloader in a while so I can't really say how trust worthy it is now. But maybe you can find an alternative software for that.
Plus if you want I can make a list of sites I check for OSTs. Some are like this one that you need to use some software for downloading. But when I can't find stuff on better ones I use them.
